1. Field of the Invention
This invention pertains to large diameter fluid flow ducts for use in ocean engineering and the like. More particularly, it pertains to such a duct which is defined of a flexible material such as high density polyethylene.
2. Review of the Prior Art and Its Problems
Many sophisticated proposals have been made in the field of ocean engineering which call for the use of large diameter vertical ducts of great length extending from at or near the ocean surface to lower ends unconnected to the ocean floor. These proposals include concepts for ocean thermal energy conversion and for mariculture.
The ocean thermal energy conversion (OTEC) concepts propose to use the difference in thermal energy levels between warm surface water and colder deep water to generate electricity, for example. The available energy level difference is low and so these proposals rely on the use of very large quantities of warm and cold water, and call for the necessary large volumes of deep ocean cold water to be brought to the water surface through very large vertical ducts of great length. The rate of water flow through the ducts, called "upwelling ducts" or "riser pipes", would be low, and thus the pressure differential across the walls of the duct would be correspondingly low, especially if the water in the duct is not carried far above the ocean surface before being discharged from the duct.
Any structure which extends vertically for any significant distance in the ocean will encounter at least one ocean current. Currents impose drag forces upon such structure. The larger the structure, the greater its profile (effective area) presented to the current, and therefore the greater the drag forces which a given current will impose on the structure. These drag forces impose bending loads on the structure, and the longer the structure, the more severe the resulting bending stresses. The problem of ocean current drag forces and of the bending stresses produced thereby is a significant difficulty in the offshore drilling industry in regard to the riser pipes which are used to conduct drilling mud during the drilling of a subsea oil or gas well from the well bore to the surface drilling platform. Current-induced bending stresses are so great in these riser pipes that the pipes must be made very heavy, i.e., with thick walls, to enable them to withstand the bending stresses. This problem is such that, because of it, offshore drilling operations today are effectively limited to water depths of about 1000 feet or so. This limit is imposed principally by the riser pipes. An OTEC cold water riser pipe may have a length of 2000 feet or more.
In commonly-owned copending patent application Ser. No. 886,904 filed Mar. 15, 1978, there is described an OTEC upwelling pipe composed of individual lengths of synthetic pipe interconnected by flexible couplings. The pipe assembly is disposed about an elongate tensile core element which carries the weight of the entire pipe assembly. As described in commonly-owned copending application Ser. No. 886,907 filed Mar. 15, 1978, the tensile core of the pipe assembly can also carry the weight of a stabilizing mass suspended below the lower open end of the upwelling duct as such.
These arrangements meaningfully address the problem of bending stresses induced in an upwelling pipe due to ocean current drag forces. The presence of flexible couplings in the pipe, and the presence of spiders across the interior of the pipe between the tensile core and the several pipe lengths, complicates the construction of the overall assembly and presents impediments to the smooth flow of water through the assembly.
In addition to the problem of current-induced stresses in an OTEC riser pipe, there is also the problem of stresses applied to the pipe, especially at its upper end, by motions of the floating structure to which the pipe is connected in use. It has also been proposed to connect the upper end of an OTEC upwelling pipe to the surface-floating structure by a connection which isolates angular motion, as in pitch and roll, of the floating structure from the pipe. This proposal is described in concurrently filed application Ser. No. 935,591.
It is therefore seen that a need exists for an ocean engineering water-flow pipe assembly of great length in which the problem of current-induced bending stresses is effectively resolved, which is simple to construct and to install, and which does not cause undesired resistance to water flow through the pipe.